The present invention relates to electrochemical cells such as fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a porous carbon body that may be used within a fuel cell for transporting reactant, product and coolant fluids to, through and from the fuel cell, for conducting electricity from one cell to an adjacent cell, for providing a fluid barrier to gaseous reactants, for defining gaseous reactant distribution channels, and/or for providing mechanical integrity to the fuel cell.
Fuel cells are well-known and are commonly used to produce electrical energy from reducing and oxidizing reactants fluids to power electrical apparatus such as apparatus on-board space vehicles, or on-site generators for buildings. A plurality of planar fuel cells are typically arranged in a stack surrounded by an electronically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids as part of a fuel cell power plant. Each individual fuel cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
It is known to utilize one component of a fuel cell to assist in the accomplishment of a variety of water management and related tasks. Such a component is typically formed of a porous carbon body and is commonly referred to under various names including xe2x80x9ccooler platexe2x80x9d, xe2x80x9cwater transport platexe2x80x9d, xe2x80x9cseparator platexe2x80x9d, xe2x80x9cbi-polar platexe2x80x9d, xe2x80x9cend platexe2x80x9d, among other names. For example, in U.S. Pat. No. 6,024,848 that issued on Feb. 15, 2000 to Dufner et al., which patent is owned by the assignee of all rights in the present invention and which patent is hereby incorporated herein by reference, a water transport plate is shown that defines a plurality of coolant water feed channels on a planar surface of the plate and on an opposed surface a network of reactant gas distribution channels is defined.
Such a water transport plate is a typical porous carbon body and the plate must perform a variety of functions. It must transport water from coolant channels through the body to gaseous reactant channels to humidify a reactant fluid within the gas reactant channels; it must remove product water generated at the cathode electrode across the body into the coolant water channels to prevent flooding of the cathode electrode; it must form a gaseous barrier to prevent mixing of fuel and oxidant reactant fluids on opposed sides of the plate; it must conduct electricity or electrons from one cell to an adjacent fuel cell in a fuel cell stack assembly; it must conduct waste heat generated at the cathode away from the cathode to the coolant fluid; it may provide a distribution network for oxidant and reducing fluid reactants; and, it must provide mechanical support and integrity to the fuel cell.
Therefore, a porous carbon body that makes up such a water transport plate must be porous, wettable to water, have a high rate of water permeability, have a high bubble pressure, be a good electrical and thermal conductor, have good compressive and flexural strength, and the porous carbon body must be chemically stable in the environment of an operating PEM fuel cell. Some of these qualities require characteristics that are inconsistent with characteristics appropriate for other such qualities. For example, to increase bubble pressure to thereby enhance a gaseous seal between gaseous oxidant and fuel reactants on opposed sides of the porous carbon body, it is appropriate to have a small mean pore size of the pores within the body. However, to enhance permeability of the body to coolant or product water, it is desirable to have a large mean pore size. Similarly, a high porosity, or percent open pore volume, is appropriate for enhancing flow of water through the porous carbon body, however a high porosity is detrimental to both electrical conductivity and mechanical strength.
Known porous carbon bodies utilized in fuel cells have been designed to reconcile such differing requirements. For example, it is known to render pores of a carbon body hydrophilic through incorporation of a hydrophilic rendering compound onto an interior surface of the pores, wherein the compound is a low solubility metal, such as shown in U.S. Pat. No. 6,258,476 that issued on Jul. 10, 2001 to Cipollini, which patent is owned by the assignee of all rights in the present invention and which patent is hereby incorporated herein by reference. In U.S. Pat. No. 5,840,414 that issued on Nov. 24, 1998 to Bett et al., which patent is owned by the assignee of all rights in the present invention and which patent is hereby incorporated herein by reference, a porous carbon body is shown that achieves increased wettability by incorporation of a metal oxide into the pores of the body, wherein the metal oxide has a solubility in water of less than about 10xe2x88x926 moles per liter. While Bett et al. makes the porous carbon body more hydrophilic, the process described in Bett includes a costly, and time consuming treatment to a graphitized carbon body. Normal manufacture of graphitized carbon bodies includes a very high temperature, lengthy process in order to produce a crystallized graphite structure. For example, to graphitize a mixture of a graphite powder and a resin into a porous carbon body acceptable for use in a PEM fuel cell, it is known to first compress the mixture in a mold to establish a pre-determined porosity at about 100-500 pounds per square inch (xe2x80x9cp.s.i.xe2x80x9d) and at about 325-375 degrees Fahrenheit (xe2x80x9cxc2x0 F.xe2x80x9d), and to then heat the molded body in an inert atmosphere at about 3,600-5,400xc2x0 F. As can be easily understood, such a process is quite expensive and time consuming often taking several weeks, and hence is a substantial problem in providing a cost effective porous carbon body for a PEM fuel cell.
Another approach to producing a porous carbon body for a PEM fuel cell is disclosed in U.S. Pat. No. 5,942,347 that issued on Aug. 24, 1999 to Koncar et al. wherein the body is described as a xe2x80x9cbi-polar separator platexe2x80x9d. The plate includes 50% to about 95% by weight of a preferably carbonaceous xe2x80x9celectronically conductive materialxe2x80x9d, at least 5% by weight of a resin, and at least one hydrophilic agent wherein the conductive material, resin and hydrophilic agent are substantially uniformly dispersed throughout the separator plate. In formation of the Koncar et al. plate, the hydrophilic or wetting agent is mixed together with the electronically conductive material and resin to produce a xe2x80x9cuniform dispersionxe2x80x9d of the wetting agent, and the mixture is then molded into a plate at 500-4,000 p.s.i. and 250-800xc2x0 F. The hydrophilic or wetting agent is an oxide of titanium, aluminum or silica. Unfortunately however, the oxides of those substances are electronic insulators. Therefore, as shown in FIG. 2 of Koncar et al., as the amount of the hydrophilic agent is increased, the conductivity of the resulting porous carbon body decreases. Hence, the carbon body of Koncar et al. necessarily results in a compromise between enhancing electrical conductivity of the body and making the body wettable to water. To increase wettability of the Koncar et al. porous carbon body, electrical conductivity must be decreased.
Accordingly, there is a need for a porous carbon body for a fuel cell that may be efficiently manufactured, and that in rendering the body adequately wettable to water does not decrease electrical conductivity of the body.
The invention is a porous carbon body having an electronically conductive hydrophilic agent and method of manufacture of the body for usage in an electrochemical cell such as a fuel cell. The porous carbon body comprises an electronically conductive graphite powder in an amount of between 60%-80% by weight of the body; a carbon fiber in an amount of between 5%-15% by weight of the body; a thermoset binder in an amount of between 6%-18% by weight of the body; and, a modified carbon black electronically conductive hydrophilic agent in an amount of between 2%-20% by weight of the body, the modified carbon black being carbon having attached at least one organic group, the organic group including at least one aromatic group or a C1-C12 alkyl group, and the organic group also including at least one ionic group, one ionizable group, or a mixture of an ionic group and an ionizable group wherein the ionic or the ionizable group is a sulfonic acid or a salt thereof, wherein the at least one aromatic group or C1-C12 alkyl of the organic group is directly attached to the carbon, and the organic group is present at a level of from about 0.10 to about 4.0 micromoles/m2 of the carbon used based on the nitrogen surface area of the carbon; and, wherein the body has a mean pore size of greater than 2.0 microns, and an open porosity of greater than 25%.
The porous carbon body having an electronically conductive hydrophilic agent may be efficiently made by mixing together an electronically conductive graphite powder in an amount between 60%-80% by weight of the mixture, a carbon fiber in an amount of between 5%-15% by weight of the mixture, a thermoset binder in an amount of between 6%-18% by weight of the mixture, and the modified carbon black electronically conductive hydrophilic agent in an amount between 2%-20%; then simultaneously compressing and heating the mixture in a mold at a pressure of between 250-1,000 p.s.i. and at a temperature of between 300xc2x0 F.-450xc2x0 F. for between 1-30 minutes to form a body having a mean pore size of greater than 2.0 microns and an open porosity of greater than 25%.
By mixing a thermoset binder with the graphite, carbon fiber, and modified carbon black in the aforesaid proportions, compressing and heating the mixture into a molded porous carbon body having a mean pore size greater than 2.0 microns and an open porosity greater than 25%, the porous carbon body of the present invention may be efficiently manufactured without the known costly and time consuming high temperature heating undertaken to graphitize many known porous carbon bodies used in fuel cells, and without a time consuming and costly step of treating a porous carbon body after molding with a hydrophilic rendering compound. The resulting porous carbon body also exhibits appropriate bubble pressure, water permeability, electrical conductivity, thermal conductivity, compressive and flexural strength to efficiently serve as a water transport plate, separator plate or related component of a PEM fuel cell operating at 1,000-2,000 ASF for a very long time period.
Accordingly, it is a general object of the present invention to provide a porous carbon body for an electrochemical cell such as a fuel cell having an electronically conductive hydrophilic agent for a fuel cell and a method of manufacture of the body that overcomes deficiencies of prior art porous bodies for fuel cells.
It is a more specific object to provide a method of manufacturing a porous carbon body having an electronically conductive hydrophilic agent for a fuel cell without a time consuming, high temperature treatment to graphitize the body, and without a post molding step of treating the body with a hydrophilic rendering agent.
It is yet another object to provide a porous carbon body for a fuel cell having an electronically conductive hydrophilic agent that may serve as a water transport plate, a reactant fluid separator plate, a reactant flow field plate, and/or an end plate between fuel cells.
It is still a further object to provide a porous carbon body for a fuel cell having an electronically conductive hydrophilic agent that is highly wettable by water formed during operation of the fuel cell and/or by coolant water passing through the fuel cell.
It is another specific object to provide a porous carbon body for a fuel cell having an electronically conductive hydrophilic agent that has an extended useful life and does not degrade during operation of the fuel cell over long time periods.
These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.